The present invention relates to a heat exchanger, especially an exhaust heat exchanger, with tubes held at both ends in tube sheets to guide a gaseous medium and with a housing abutting the tube sheets and surrounding the tubes for guiding a liquid coolant. The tube sheet, the tubes and the housing are shaped from heat-resistant austenitic steel sheets, with the tubes welded into, and the housing welded to, the tube sheets.
A heat exchanger of this general type is known from German patent document DE 195 40 683 A1. This German patent document corresponds to co-pending U.S. patent application Ser. No. 08/743,002, filed Nov. 1, 1996.
The primary object of the invention is to design a heat exchanger of the type mentioned above such that it can be manufactured economically and advantageously in terms of welding technology.
This object is achieved by shaping the tube sheets as deep-drawn and punched parts, each of which has a circumferential wall that projects outward beyond the ends of the tubes. Each wall is welded to the housing and abuts a diffuser.
The design of the heat exchanger according to the invention permits welding in which as little heat as possible is conducted into the tube sheets, especially when welding parts to the tube sheets, so that deformation of the tube sheets by heat is reliably prevented.
In one advantageous embodiment of the invention, the housing and the tube sheets are trimmed in the vicinity of the surrounding walls after the welded connections have been made. In this fashion, a heat exchanger can be produced that maintains, with high accuracy, installation tolerances that are required especially in motor vehicles.
In another embodiment of the invention, the housing and the tube sheets are welded together by a circumferential auxiliary seam and by a sealing seam provided closer to the tube ends. The housing and tube sheets are cut to length in the vicinity of the auxiliary seam. Thus, the housing and the tube sheets form a tightly closed edge. Since the actual sealing seam can be placed relatively close to the area of a tube sheet that receives the tube ends, the possibility of crevice corrosion taking place in the area between the tube sheet and the housing is largely eliminated.
In an advantageous embodiment, a diffuser is butt-welded to the circumferential trimmed edge. As a result, an advantageous welded connection is created between the diffuser and the edge at which partial compensation for length tolerances can be made. In another embodiment of the invention, the housing has larger cross sections adjacent to the tube sheets than in the area located therebetween, in which it surrounds of the tubes at a short distance. In the area with a larger cross section, a type of water box is thus created in which the liquid coolant distributes itself over the entire cross section of the heat exchanger.
In another embodiment of the invention, the tubes are provided with externally projecting bump-shaped projections by which they are supported against one another and against the inside wall of the housing. This increases the stability of the entire heat exchanger, with the production of noise, caused by introduced vibrations or oscillations, being largely eliminated at the same time.
In another embodiment of the invention, the housing that is assembled from at least two shaped sheet-metal parts is welded by welded seams that run parallel to the tubes. Even when the welded seams are produced, only small quantities of heat are conducted into the tubes or tube sheets. Provision is advantageously made such that the welded seams are straight butt welds in the vicinity of the largest housing cross section. The result is a largely smooth outer contour of the heat exchanger in this area. Provision is also made for the welded seams in the vicinity of the smaller housing cross section to be in the form of crimped seams. In this way it is possible to allow the welded seam to be on one level throughout. It is especially advantageous when the housing parts are welded while being elastically pretensioned. The elastic pretensioning, which causes the edges of the sheet metal parts to be joined to fit together tightly, also ensures that the housing parts, following welding, abut one another in the vicinity of the smaller housing cross section under pretension against the bumps on the outer tubes of the tube bundle so that the risk of noise being produced by vibration is further reduced.
This elastic pretensioning is provided mainly in the vicinity of the crimped seams.
In another embodiment of the invention, the housing is made of a tubular section with two end areas which are expanded. This creates a one-piece housing jacket that is easy to handle during assembly.
In another embodiment of the invention, the tubes are welded to the tube sheets by laser beam welding or electron beam welding. In order to be able to perform this welding precisely, the tube sheets are provided with positioning aids located at predetermined positions. As a result, it is possible to align the tube sheets exactly with the tubes during welding.
In another embodiment of the invention, the tubes are inserted into the tube sheets with a specified excess length and the welded seams are made immediately adjacent to the outside walls of the tubes using laser or electron beams striking at an angle. This is especially advantageous in larger heat exchangers, in other words in heat exchangers with a tube bundle consisting of a plurality of tubes. With a large number of tubes, it is almost impossible to prevent the tube sheets from being slightly deformed when the tubes are inserted into the holes in the sheets. This deformation can be overcome by welding with beams that strike at an angle so that the risk of incomplete or leaky welds is largely avoided.
In another embodiment, the housing is provided, in one area with a larger cross section, with a connecting tube to supply the coolant and in another area with a larger cross section with a connecting tube for removing coolant. The housing is further provided with beams, directed outward, to which the connecting tubes are welded. This also produces an economical solution that is advantageous from the welding standpoint for attaching the connections for the coolant.
Additional features and advantages of the invention will be apparent from the following description of preferred embodiments.